150 



Schafer's Essentials of Histology, 1920, fig. 174a). These figures are to be 

 regarded as slightly diagrammatic, and represent Schafer's interpretation of 

 actual photographs. In the thickest of these, which is regarded as a contracted 

 condition of the other two, a faint Hensen's line has been indicated ; yet no 

 such structure is visible in this portion in the original microphotographs (fig. 3a 

 of the 1891 paper). 



From the above account it will appear that Schafer's conclusions rest on 

 no very secure basis ; there is no evidence to show that what he regarded as 

 contracted sarcostyles were really such. When on the contrary, contracted 

 sarcostyles ore observed, we find that a movement of the dark material has taken 

 place, just as occurs in other striated fibres (fig. 8, pi. xii.). There is also some 

 evidence to show that the degree of contraction of individual sarcomeres is less 

 in wing muscles than in other fibres. The length of a contractile wave in 

 wing muscle is not known; but unless it is very minute indeed, a contraction 

 of the individual sarcomeres to less than half their length, such as would be 

 the case if Schafer's interpretation of fig. 3a be correct, would immediately result 

 in a collapse of the thorax of the insect. While the rate of contraction of wing 

 muscle tissue may be more rapid than that of other striated fibres, there is 

 no evidence to show that the degree of the contraction is so extensive as in 

 other striated fibres. 



I wish, in conclusion, to thank Mr. W\ Fuller for the loan of some excellent 

 gold chloride preparations. From one of these, fig. 20, pi. xiv., has been 

 drawn. 



Summary. 



(1) The striations and Krause's membranes of muscle fibres are disposed 

 not as transverse discs, but in the form of a double spiral (helicoid) which 

 stretches from one end of the fibre to the other. 



(2) In wing-moving muscles of insects the sarcostyles are developed each 

 from one cell ; in all other fibres they are of intracellular formation. The 

 similarity between ordinary muscle fibres and wing muscles is the result of a 

 remarkable process of evolutionary convergence. 



(3) A sarcomere is constituted as follows: — In connection with Krause's 

 membrane is a minute quantity of residual hyaloplasm which communicates 

 through minute tubules in the clear region with the material of the striation 

 (movable hyaloplasm). At contraction the surface tension of the hyaloplasm 

 evidently decreases, and the movable hyaloplasm is rapidly drawn down the 

 tubules, having retreated from the doubly refracting spongy network which 

 mechanical conditions show must be present within the sarcomere. There is then 

 a true reversal of striations. At relaxation there is an increase of surface tension, 

 and the hyaloplasm is again drawn up the minute capillary tubes. 



(4) Schafer's and Engelmann's interpretation of muscle action can no longer 

 be accepted. 



(5) Ordinary muscle nuclei, as well as motor end plate nuclei, have an 

 intimate relation with Krause's membrane. 



References. 



Bernstein (1901)— Arch. f. ges. Physiol., vol. 85, p. 271. 

 Bowman. W. (1840) — Phil. Trans. Roy. Soc. London, vol. 130. pt. 2. p. A57. 

 Dobie, W. M. (1849)— Annals of Nat. Science, Second Series, vol. 8, p. 109. 

 Engelmann, T. W. (1878)— Pfliiger's Archiv., vol. 18, p. 1. 

 (1881)— /&/(/., vol. 23, p. 571. 



